Actinopterygii

Actinopterygii (/ˌæktɪnɒptəˈrɪdʒiaɪ/; from actino- 'having rays', and Ancient Greek πτέρυξ (ptérux) 'wing, fins'), members of which are known as ray-finned fish or actinopterygians, is a class of bony fish[2] that comprise over 50% of living vertebrate species.[3] They are so called because of their lightly built fins made of webs of skin supported by radially extended bony spines, as opposed to the bulkier, fleshy lobed fins of the sister class Sarcopterygii (lobe-finned fish). Resembling folding fans, the actinopterygian fins can change shape easily and provide superior thrust-to-weight ratios per movement compared to sarcopterygian and chondrichthyian fins. The fin rays attach directly to the proximal or basal skeletal elements, the radials, which represent the articulation between these fins and the internal skeleton (e.g., pelvic and pectoral girdles).

Ray-finned fish
Temporal range:
Late Silurian–Present,
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Superclass: Osteichthyes
Class: Actinopterygii
Klein, 1885
Subclasses

The vast majority (~99%) of actinopterygians are teleosts. By species count, they dominate the subphylum Vertebrata, and constitute nearly 99% of the over 30,000 extant species of fish.[4] They are the most abundant nektonic aquatic animals and are ubiquitous throughout freshwater and marine environments from the deep sea to subterranean waters to the highest mountain streams. Extant species can range in size from Paedocypris, at 8 mm (0.3 in); to the massive ocean sunfish, at 2,300 kg (5,070 lb); and to the long-bodied oarfish, at 11 m (36 ft).

Characteristics
Anatomy of a typical ray-finned fish (cichlid)
A: dorsal fin, B: fin rays, C: lateral line, D: kidney, E: swim bladder, F: Weberian apparatus, G: inner ear, H: brain, I: nostrils, L: eye, M: gills, N: heart, O: stomach, P: gall bladder, Q: spleen, R: internal sex organs (ovaries or testes), S: ventral fins, T: spine, U: anal fin, V: tail (caudal fin). Possible other parts not shown: barbels, adipose fin, external genitalia (gonopodium)

Ray-finned fishes occur in many variant forms. The main features of typical ray-finned fish are shown in the adjacent diagram. The swim bladder is a more derived structure than the lung and used for buoyancy.[5] Except from the bichirs, which just like the Sarcopterygii (lobe-finned fish) have retained the ancestral condition of ventrally budding lungs, the swim bladder in ray-finned fishes derives from a dorsal bud above the foregut.[6][5] In early forms the swim bladder could still be used for breathing, a trait still present in Holostei (bowfins and gars).[7] In some fish, like the arapaima, the swim bladder has been modified for breathing air again,[8] and in other lineages it have been completely lost.[9]

Ray-finned fishes have many different types of scales; but all teleosts have leptoid scales. The outer part of these scales fan out with bony ridges, while the inner part is crossed with fibrous connective tissue. Leptoid scales are thinner and more transparent than other types of scales, and lack the hardened enamel or dentine-like layers found in the scales of many other fish. Unlike ganoid scales, which are found in non-teleost actinopterygians, new scales are added in concentric layers as the fish grows.[10]

Teleosts also differ from other ray-finned fishes in having gone through a whole-genome duplication (paleopolyploidy).[11][12]

Body shapes and fin arrangements
Further information: Fish fin and Diversity of fish

Ray-finned fish vary in size and shape, in their feeding specializations, and in the number and arrangement of their ray-fins.

Reproduction
Three-spined stickleback (Gasterosteus aculeatus) males (red belly) build nests and compete to attract females to lay eggs in them. Males then defend and fan the eggs. Painting by Alexander Francis Lydon, 1879

In nearly all ray-finned fish, the sexes are separate, and in most species the females spawn eggs that are fertilized externally, typically with the male inseminating the eggs after they are laid. Development then proceeds with a free-swimming larval stage.[13] However other patterns of ontogeny exist, with one of the commonest being sequential hermaphroditism. In most cases this involves protogyny, fish starting life as females and converting to males at some stage, triggered by some internal or external factor. Protandry, where a fish converts from male to female, is much less common than protogyny.[14]

Most families use external rather than internal fertilization.[15] Of the oviparous teleosts, most (79%) do not provide parental care.[16] Viviparity, ovoviviparity, or some form of parental care for eggs, whether by the male, the female, or both parents is seen in a significant fraction (21%) of the 422 teleost families; no care is likely the ancestral condition.[16] The oldest case of viviparity in ray-finned fish is found in Middle Triassic species of †Saurichthys.[17] Viviparity is relatively rare and is found in about 6% of living teleost species; male care is far more common than female care.[16][18] Male territoriality "preadapts" a species for evolving male parental care.[19][20]

There are a few examples of fish that self-fertilise. The mangrove rivulus is an amphibious, simultaneous hermaphrodite, producing both eggs and spawn and having internal fertilisation. This mode of reproduction may be related to the fish's habit of spending long periods out of water in the mangrove forests it inhabits. Males are occasionally produced at temperatures below 19 °C (66 °F) and can fertilise eggs that are then spawned by the female. This maintains genetic variability in a species that is otherwise highly inbred.[21]

Classification and fossil record
See also: Evolution of fish

Actinopterygii is divided into the classes Cladistia and Actinopteri. The latter comprises the subclasses Chondrostei and Neopterygii. The Neopterygii, in turn, is divided into the infraclasses Holostei and Teleostei. During the Mesozoic (Triassic, Jurassic, Cretaceous) and Cenozoic the teleosts in particular diversified widely. As a result, 96% of living fish species are teleosts (40% of all fish species belong to the teleost subgroup Acanthomorpha), while all other groups of actinopterygians represent depauperate lineages.[22]

The classification of ray-finned fishes can be summarized as follows:

  • Cladistia, which include bichirs and reedfish
  • Actinopteri, which include:
    • Chondrostei, which include Acipenseriformes (paddlefishes and sturgeons)
    • Neopterygii, which include:
      • Teleostei (most living fishes)
      • Holostei, which include:
        • Lepisosteiformes (gars)
        • Amiiformes (bowfin)

The cladogram below shows the main clades of living actinopterygians and their evolutionary relationships to other extant groups of fishes and the four-limbed vertebrates (tetrapods).[23][24] The latter include mostly terrestrial species but also groups that became secondarily aquatic (e.g. Whales and Dolphins). Tetrapods evolved from a group of bony fish during the Devonian period.[25] Approximate divergence dates for the different actinopterygian clades (in millions of years, mya) are from Near et al., 2012.[23]

Vertebrates
Jawed vertebrates
Euteleostomi
Sarcopterygii
Rhipidistia
Tetrapods
Amniota

Sauropsids (reptiles, birds)

Mammals

Amphibians

Lungfish

Actinistia

Coelacanths

(lobe‑fins)
Actinopterygii
Cladistia

Polypteriformes (bichirs, reedfishes)

Actinopteri
Chondrostei

Acipenseriformes (sturgeons, paddlefishes)

Neopterygii
Holostei

Lepisosteiformes (gars)

Amiiformes (bowfins)

275 mya

Teleostei

310 mya
360 mya
400 mya
('bony fish')

Cartilaginous fishes (sharks, rays, ratfish)

Jaw-less fishes (hagfish, lampreys)

The polypterids (bichirs and reedfish) are the sister lineage of all other actinopterygians, the Acipenseriformes (sturgeons and paddlefishes) are the sister lineage of Neopterygii, and Holostei (bowfin and gars) are the sister lineage of teleosts. The Elopomorpha (eels and tarpons) appear to be the most basal teleosts.[23]

The earliest known fossil actinopterygian is Andreolepis hedei, dating back 420 million years (Late Silurian), remains of which have been found in Russia, Sweden, and Estonia.[26] Crown group actinopterygians most likely originated near the Devonian-Carboniferous boundary.[27] The earliest fossil relatives of modern teleosts are from the Triassic period (Prohalecites, Pholidophorus),[28][29] although it is suspected that teleosts originated already during the Paleozoic Era.[23]

Chondrostei
Atlantic sturgeon
 Chondrostei (cartilage bone) is a subclass of primarily cartilaginous fish showing some ossification. Earlier definitions of Chondrostei are now known to be paraphyletic, meaning that this subclass does not contain all the descendants of their common ancestor. There used to be 52 species divided among two orders, the Acipenseriformes (sturgeons and paddlefishes) and the Polypteriformes (reedfishes and bichirs). Reedfish and birchirs are now separated from the Chondrostei into their own sister lineage, the Cladistia. It is thought that the chondrosteans evolved from bony fish but lost the bony hardening of their cartilaginous skeletons, resulting in a lightening of the frame. Elderly chondrosteans show beginnings of ossification of the skeleton, suggesting that this process is delayed rather than lost in these fish.[30] This group had once been classified with the sharks: the similarities are obvious, as not only do the chondrosteans mostly lack bone, but the structure of the jaw is more akin to that of sharks than other bony fish, and both lack scales (excluding the Polypteriforms). Additional shared features include spiracles and, in sturgeons, a heterocercal tail (the vertebrae extend into the larger lobe of the caudal fin). However the fossil record suggests that these fish have more in common with the Teleostei than their external appearance might suggest.[30]
Neopterygii
Atlantic salmon
 Neopterygii (new fins) is a subclass of ray-finned fish that appeared somewhere in the Late Permian. There were only few changes during its evolution from the earlier actinopterygians. Neopterygians are a very successful group of fishes because they can move more rapidly than their ancestors. Their scales and skeletons began to lighten during their evolution, and their jaws became more powerful and efficient. While electroreception and the ampullae of Lorenzini is present in all other groups of fish, with the exception of hagfish, neopterygians have lost this sense, though it later re-evolved within Gymnotiformes and catfishes, who possess nonhomologous teleost ampullae.[31]
Fossil of the Devonian †cheirolepidiform †Cheirolepis canadensis
Fossil of the Carboniferous †elonichthyiform †Elonichthys peltigerus
Fossil of the Permian †aeduelliform †Aeduella blainvillei
Fossil of the Permian †palaeonisciform †Palaeoniscum freieslebeni
Fossil of the Triassic †bobasatraniiform †Bobasatrania canadensis
Fossil of the Triassic †perleidiform †Thoracopterus magnificus
Fossils of the Triassic †prohaleciteiform †Prohalecites sp., the earliest teleosteomorph
Fossil of the Jurassic †aspidorhynchiform †Aspidorhynchus sp.
Fossil of the Jurassic †pachycormiform †Pachycormus curtus
Fossil of the Cretaceous acipenseriform †Yanosteus longidorsalis
Fossil of the Cretaceous aulopiform †Nematonotus longispinus
Fossil of the Cretaceous †ichthyodectiform †Thrissops formosus
Fossil of the Cretaceous carangiform †Mene oblonga
Fossil of the Cretaceous pleuronectiform †Amphistium paradoxum
Fossil of a ray-finned perch (†Priscacara serrata) from the Lower Eocene about 50 million years ago
Fossil of the Miocene syngnathiform †Nerophis zapfei
Skeleton of the angler fish, Lophius piscatorius. The first spine of the dorsal fin of the anglerfish is modified so it functions like a fishing rod with a lure
Skeleton of another ray-finned fish, the lingcod
Blue catfish skeleton

Taxonomy

The listing below is a summary of all extinct (indicated by a dagger, †) and living groups of Actinopterygii with their respective taxonomic rank. The taxonomy follows Phylogenetic Classification of Bony Fishes[24][32] with notes when this differs from Nelson,[3] ITIS[33] and FishBase[34] and extinct groups from Van der Laan 2016[35] and Xu 2021.[36]

References
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  2. Kardong, Kenneth (2015). Vertebrates: Comparative Anatomy, Function, Evolution. New York: McGraw-Hill Education. pp. 99–100. ISBN 978-0-07-802302-6.
  3. Nelson, Joseph S. (2016). Fishes of the World. John Wiley & Sons. ISBN 978-1-118-34233-6.
  4. (Davis, Brian 2010).
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  7. A single-cell atlas of West African lungfish respiratory system reveals evolutionary adaptations to terrestrialization
  8. Morphology of the Amazonian Teleost Genus Arapaima Using Advanced 3D Imaging
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  33. "Actinopterygii". Integrated Taxonomic Information System. Retrieved 3 April 2006.
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  37. In Nelson, Polypteriformes is placed in its own subclass Cladistia.
  38. In Nelson and ITIS, Syngnathiformes is placed as the suborder Syngnathoidei of the order Gasterosteiformes.
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